In the United States, the In-Band On-Channel (IBOC) technology, referred to as HD Radio™, has been selected by the Federal Communication Commission to be the standard for simulcast digital programming along with traditional analog audio at the same frequency band. In other jurisdictions, other standards have been adopted. For example, several European Union countries have implemented Digital Audio Broadcasting (DAB) for FM broadcasts and Digital Radio Mondiale (DRM) for AM broadcasts.
HD Radio™ IBOC technology is proprietary to iBiquity Digital Corporation, which develops and licenses the various components and technologies required for HD Radio™. As such, the instructions provided with the equipment and technology, and training provided by iBiquity forms the common general knowledge in this field and allows the technology to be put into practice.
IBOC makes use of orthogonal frequency division multiplexing (OFDM) signalling. While it can be shown that OFDM provides substantial benefits in digital wireless communications, one of its main disadvantages is the fact that in the time domain the multitude of subcarriers add constructively or destructively almost at random. This produces a time domain signal with widely varying power.
A high level combined broadcast transmission system takes the output from a full power analog transmitter and combines it with the signal from a linear digital transmitter before sending it to the antenna. In this case, the digital power amplifier must be able to handle the full range of the digital signal's power fluctuation. In order to make IBOC deployments economically feasible, peak-to-average power (PAPR) algorithms have been developed to reduce the power peaks in the digital signal. For example, U.S. Pat. No. 6,128,350 to Shastri et al., and US Patent Publication No. 2005/0169411 to Kroeger both describe PAPR algorithms for use on the digital signal in lBOC systems. These standard algorithms effectively brings the original 12 dB PAPR under 8 dB by solely operating on the digital signal.
Conversely, a low level combined broadcast transmission system consists of a single combined power amplification chain. In this case the analog and digital signals are added as digital complex baseband signals. The addition of the analog signal to the digital signal generates a hybrid signal and alters the power characteristics of the combined signal.
FIGS. 1A-1C illustrate a complex plane, where the X-axis reflects the baseband signal's real (or in phase—I) component and the Y-axis represents the signal's imaginary (or quadrature—Q) component. As shown in FIG. 1A, the output of the FM modulation process produces a constant envelope signal with varying phase. At baseband, this signal is represented as a vector 120 in the complex plane with constant amplitude, which is represented by a circle 107. Any given sample point can be represented by an additive vector 110, which represents the possible amplitude and phase of the additive digital signal.
In theory, as shown in FIG. 1B, the standard PAPR reduction scheme creates a circle 140 having a radius that is equal to or less than the distance between the constant FM signal level and the maximum desired peak threshold 112. Only sample points that fall within this circle 140 can be certain not to add to the analog signal 102 constructively, and thus do not require correction. However, all sample points that fall outside of the circle 140 will require correction to a point within the circle 140. For example, sample points 150, 151 and 152, defined by digital vectors 110A, 110B and 110C, respectively, will all require correction to be within the confines of circle 140.
However, this form of peak detection results in the digital signal being unnecessary corrected in a variety of circumstances and large corrections being applied when much less correction is required. For example, as shown in FIG. 1C, sample point 152 defined by digital vector 110B would be corrected by correction vector 160, when in fact no correction would be necessary, since the actual sample point 152 would fall below the maximum desired threshold 112.
Sample point 150, defined by digital vector 110C, illustrates the situation where a large correction would be applied to a sample point requiring only a small correction. In this example, digital vector 110C projects to a point 150 that is just beyond the maximum desired peak threshold 112. In the standard PAPR reduction scheme, a large correction 161 would be applied to this vector 110C to bring the sample point into the circle 140, whereas in reality a small correction function could have been used to bring the sample point 150 below the maximum desired peak threshold 112. Only in circumstances where the digital vector could add to the analog vector in-phase to generate a maximum peak does the current PAPR reduction scheme apply proper clipping or correction. For example, sample point 151 defined by digital vector 110A generates a maximum peak, in which a proper correction vector 162 is applied.
Accordingly, there is a need to develop a method that does not unnecessarily reduce peaks in power that fall within the maximum desired peak threshold and therefore overcomes the limitations of the prior art.